http://www.buildingscience.com/doctypes/enclosures-that-work/etw-high-r-value-enclosure-assemblie
From what I gather from published models a standard fiberglass or cellulose filled wall bounded by drywall and a cladding, convective air and moisture loop can cause degradation of the whole wall r-value even if the envelop is sealed and the blower door shows it good. The cavity of the wall is ‘two dimensional” and the flow through the wall one dimensional or “hygrothermal”. Fiberglass batt, and both blown and sprayed cellulose are air permeable materials allowing possible air paths between the interior and exterior as well as convective looping in the insulation. Although densepack cellulose has less air permeance it does not control air leakage. Rain leakage into the enclosure is the leading cause of premature building enclosure failure. Air need not leak straight through an assembly to cause moisture problems; it can also leak from the inside, through the wall, and back to the inside; or it can leak from the outside, through the wall, and back to the outside. Condensation within the studspace is possible if this type of airflow occurs, depending on the weather conditions. This insulation design is highly moisture vapor permable and difficult to control by a vapor barrier. It can cause mold and mildew and other health issues. If you vapor seal both sides if this cavity with no ventilation it can reduce the cellulose ability to dry, this material otherwise has good drying ability. This wall system is difficult to air seal adequately and prone to air leakage related condensation and energy losses. 2x 4 on 16 oc has more interfaces and cavities compared to 2 x 6 so it’s efficiency is lower.
A spray foam filled wall high density foam (2.0 pcf) ranges between R-5.5-R-6.5 per inch for the aged R-value, and low density foam (0.5pcf) has an R-value of approximately R-3.6/inch…. Since high density foam is generally installed short of the cavity to avoid trimming, the installed insulation R-value is approximately R-30 (using R-6/inch). Low density is generally installed deliberately overflowing the cavity and trimmed off resulting in an R-value of approximately R-21. The R-value of the high density spray foam wall decreases from an installed R-value of R-30 to approximately R-20, a decrease of R-10 because of thermal bridging. The low density spray foam wall decreases from an installed insulation R-value of 21 to a whole wall R-value of approximately R-16. Air Leakage – Both low density and high density foam form an air barrier decreasing thermal losses through air leakage. Air leakage is still common under the bottom plate and at the rim joist if these areas are not detailed correctly. Air leakage is significantly minimized by installing spray foam insulation in the studspace since both low density and high density spray foam act as an air barrier. This increases the durability of the wall system considerably over standard construction. High density (2.0 pcf) foam forms a vapor control layer reducing vapor movement through the enclosure, minimizing the potential for wintertime vapor condensation and summertime inward vapor drive. Low density foam allows moisture vapor movement through the foam so other methods of vapor control such as poly, kraft paper, or vapor barrier paint may be required based on the geographic location. Both of the spray foam walls dry relatively slowly if water enters the enclosure, since they do not experience convective looping and air movement similar to air permeable insulations. Spray foam does not provide any buffering capacity or redistribution. Foam is relatively moisture tolerant and will be able to dry given enough time. Ventilation behind vapor impermeable claddings and interior components (e.g. kitchen cabinets) can encourage drying. The primary durability risks associated with these wall assemblies involve moisture damage related to rain water penetration. Both air leakage and vapor diffusion durability is significantly increased with spray foam but some vapor control may be necessary with low density spray foam in cold climates. Spray foam significantly reduces risks of poor air tightness detailing of the exterior sheathing or interior drywall.
It may be possible to use spray foam insulation in combination with another insulation strategy to maximize the R-value gained with the spray foam insulation.
Begs the question what combination, cellulose and foam (low or high) and what does the wall assy look like?
PS: If anyone has a more up to date design guide please post it.
From what I gather from published models a standard fiberglass or cellulose filled wall bounded by drywall and a cladding, convective air and moisture loop can cause degradation of the whole wall r-value even if the envelop is sealed and the blower door shows it good. The cavity of the wall is ‘two dimensional” and the flow through the wall one dimensional or “hygrothermal”. Fiberglass batt, and both blown and sprayed cellulose are air permeable materials allowing possible air paths between the interior and exterior as well as convective looping in the insulation. Although densepack cellulose has less air permeance it does not control air leakage. Rain leakage into the enclosure is the leading cause of premature building enclosure failure. Air need not leak straight through an assembly to cause moisture problems; it can also leak from the inside, through the wall, and back to the inside; or it can leak from the outside, through the wall, and back to the outside. Condensation within the studspace is possible if this type of airflow occurs, depending on the weather conditions. This insulation design is highly moisture vapor permable and difficult to control by a vapor barrier. It can cause mold and mildew and other health issues. If you vapor seal both sides if this cavity with no ventilation it can reduce the cellulose ability to dry, this material otherwise has good drying ability. This wall system is difficult to air seal adequately and prone to air leakage related condensation and energy losses. 2x 4 on 16 oc has more interfaces and cavities compared to 2 x 6 so it’s efficiency is lower.
A spray foam filled wall high density foam (2.0 pcf) ranges between R-5.5-R-6.5 per inch for the aged R-value, and low density foam (0.5pcf) has an R-value of approximately R-3.6/inch…. Since high density foam is generally installed short of the cavity to avoid trimming, the installed insulation R-value is approximately R-30 (using R-6/inch). Low density is generally installed deliberately overflowing the cavity and trimmed off resulting in an R-value of approximately R-21. The R-value of the high density spray foam wall decreases from an installed R-value of R-30 to approximately R-20, a decrease of R-10 because of thermal bridging. The low density spray foam wall decreases from an installed insulation R-value of 21 to a whole wall R-value of approximately R-16. Air Leakage – Both low density and high density foam form an air barrier decreasing thermal losses through air leakage. Air leakage is still common under the bottom plate and at the rim joist if these areas are not detailed correctly. Air leakage is significantly minimized by installing spray foam insulation in the studspace since both low density and high density spray foam act as an air barrier. This increases the durability of the wall system considerably over standard construction. High density (2.0 pcf) foam forms a vapor control layer reducing vapor movement through the enclosure, minimizing the potential for wintertime vapor condensation and summertime inward vapor drive. Low density foam allows moisture vapor movement through the foam so other methods of vapor control such as poly, kraft paper, or vapor barrier paint may be required based on the geographic location. Both of the spray foam walls dry relatively slowly if water enters the enclosure, since they do not experience convective looping and air movement similar to air permeable insulations. Spray foam does not provide any buffering capacity or redistribution. Foam is relatively moisture tolerant and will be able to dry given enough time. Ventilation behind vapor impermeable claddings and interior components (e.g. kitchen cabinets) can encourage drying. The primary durability risks associated with these wall assemblies involve moisture damage related to rain water penetration. Both air leakage and vapor diffusion durability is significantly increased with spray foam but some vapor control may be necessary with low density spray foam in cold climates. Spray foam significantly reduces risks of poor air tightness detailing of the exterior sheathing or interior drywall.
It may be possible to use spray foam insulation in combination with another insulation strategy to maximize the R-value gained with the spray foam insulation.
Begs the question what combination, cellulose and foam (low or high) and what does the wall assy look like?
PS: If anyone has a more up to date design guide please post it.
